In construction applications it is often necessary or desirable to anchor into an existing concrete structure. In many construction applications the anchor is intended to be permanent to tie in a newer structure with an existing concrete structure. In such permanent anchoring applications, removal of the anchors is generally not a concern. Thus, the removal of such permanent anchors from the concrete will undoubtedly result in damage to the surface of the concrete.
A number of construction applications currently exist in which it is desirable to provide an anchor into an existing concrete structure that is removable. An example of such an application is present in the commercial construction business in which concrete tilt-wall construction is often employed. In a tilt-wall construction, forms are usually anchored into an existing concrete floor in the shape of the wall. A release agent is applied to the floor and the forms and concrete mix is poured into the forms. When the concrete has hardened, the forms are removed and the wall is lifted from the floor and positioned upright using a crane.
A number of “removable” anchoring systems currently exist for use with concrete. The most common of such systems include either a nail or screw, both of which require pre-drilling of a pilot hole. When a standard nail, such as an 8 common, 8 duplex, 16 common, 16 duplex or the like, is utilized, the pilot hole is usually drilled to be a diameter slightly smaller than the diameter of the nail so as to provide a tight frictional fit between the nail and the concrete when the nail is driven into the hole. A common problem with such applications is that the surface of the concrete is usually damaged as the nail is removed from the concrete.
The most common type of damage to the surface of the concrete is known as spawling, which is caused when the surface of the concrete surrounding the pilot hole is pulled upward by friction with the nail as the nail is pulled upward and breaks away leaving an indentation or pit in the surface of the concrete. When the diameter of the pilot hole is slightly smaller than the diameter of the nail, spawling usually occurs around the entire circumference of the pilot hole. Spawling in a concrete surface is undesirable as it creates an unattractive surface and patching a spawled surface is difficult as the patch will usually flake off.
In an attempt to reduce or eliminate spawling, many contractors will drill a pilot hole having a diameter slightly larger than the diameter of the nail that is being utilized. Because the diameter of the hole is larger than the nail diameter, it is then necessary to insert a softer (than the nail and the concrete) material, such as lawn trimmer cord or a wire, into the hole to provide the frictional fit between the nail and the concrete. Insertion of the softer material is extremely time consuming and requires that the contractor have an ample supply of the material on hand. Furthermore, spawling often still occurs when the nail is removed, as the softer material is usually located on one side of the nail and the opposite side is pressed up against the concrete. When the nail is removed, spawling occurs on the side of the concrete which the nail is pressed against. In some instances contractors will use a specially designed nail that includes a section of the nail shank near the end of the nail that protrudes outward. The outward protrusion provides frictional contact between the nail and the wall of the pilot hole and functions in much the same manner as the softer material described above. Such specially designed nails are extremely expensive when compared to standard nails, and also result in the same one-sided spawling discussed above.
As an alternative to nails, many contractors now use a screw anchor when a removable anchor is desired. An example of a screw anchoring system is currently marketed by “Tapcon”. This system includes a drill bit, a hex (or Phillips) driver and sleeve, and a screw. In operation a contractor will drill a pilot hole with a drill bit that is smaller than the diameter of the screw. The contractor then slides the sleeve over the drill bit so that the hex driver is powered by the drill. The contractor then uses the driver to drive the screw into the pilot hole. When it is desired to remove the screw the contractor reverses the driver. While this system does reduce spawling to the concrete surface when the screw can be reversed out of the pilot hole, some spawling often does still occur when the screw is reversed, and a number of other disadvantages exist. Screws are relatively expensive (approx. $0.18 each) when compared to the cost of standard nails (less than. $0.01 each). The wear on the contractor's drill is significant as it is used not only to drill the pilot hole, which necessitates very little torque, but also to drive and reverse the screw, which necessitates considerable torque. The time requirement of driving and reversing the screws is significant. In addition, the heads of the screws often strip out, making reversal of the screws impossible and resulting in substantial spawling to the concrete surface when the screw must be forcefully extracted from the concrete. Therefore, it would be beneficial to provide a low cost concrete anchoring system that can be removed without resulting in spawling to the surface of the concrete.